This invention is related to a brake pressure control unit for a hydraulic fluid-actuatable vehicle brake system of the type including a spring device for load-sensitively pivoting the pressure control unit and wherein the spring device is pivotably mounted to a vehicle member for generating input motions relative to the brake pressure control unit. The pressure control unit is provided with at least one valve with a control piston being pressure-sensitively slidable in the direction of closure of the valve against the force of the spring device. These systems typically provide for the spring device to be mounted to a swivelling transmission lever, which is connected to the control piston of the valve. A pilot device through which the spring force of the spring device is made variable in response to the load of the vehicle is connected to the spring device and includes a first pilot element connected to the vehicle member providing the pivotal input motion, a second pilot element connected to the transmission lever, and a spring positioned between the two pilot elements.
A brake pressure control unit of this type is disclosed in the European patent applicatio No. 0,029,398 which includes a pilot device substantially comprising a first pilot element connected to an unsprung member of the vehicle, a second pilot element connected through a tension spring device to the transmission lever which is supported at the housing, and an auxillary spring is arranged between the first and second pilot elements. During mounting of this brake pressure control unit to a vehicle, the housing is fastened to a sprung member and the first pilot element is fastened to an unsprung member of the vehicle with the load-sensitive spring device locked. A removable stop ring is arranged at the first pilot element during mounting. The length of the second pilot element is varied until the end of the second pilot element is in abutment against the stop ring of the first pilot element. During this installation, the transmission lever is in the position corresponding to the open position of the valve. After mounting, the spring device is unloaded and the stop ring at the first pilot element removed. The second pilot element will thereby slide against the force of an auxially spring positioned internally between the pilot elements until an equilibrium position between the two spring forces is reached. In the event of a variation of the position of the unsprung vehicle member relative to the housing of the brake pressure control unit, an equilibrium between the spring forces will take place. Two spring elements thereby contribute to the application of the force of control transmitted by the transmission lever to the valve device of the brake pressure control unit, which can lead to reduced precision of the piloting action.
In order to improve the precision, it has heretofore been proposed in a brake pressure control unit of this type to lock the spring device during mounting of the brake pressure control unit in the vehicle in a position corresponding to a predetermined loading condition, and during this procedure, to rigidly lock together the first and the second pilot elements to form a constructional unit of a defined length. This method bears the advantage that the auxilliary spring positioned between the pilot elements does not influence the force of control. When the brake pressure control unit and the first pilot element are being fastened to the respective vehicle members with the spring device locked, the transmission lever will assume that position which corresponds to a defined open position of the valve. The effect of the auxilliary spring on the piloting of the brake pressure control unit is eliminated as a result of the two pilot elements being rigidly interconnected. Incorrect adjustment of the pilot device caused by an incorrect adjustment of the transmission lever is eliminated. This system requires that, during mounting, the vehicle or, more precisely, its axle to which one pilot element is attached must be in a predetermined loading condition which corresponds to the position of the second pilot element with respect to the transmission lever with the spring device in locked condition. Therefore, such a device presently requires that the vehicle be in a loaded state, most typically, having all wheels resting on the ground during installation of the brake pressure control unit.